Aerogels are porous solids with high surface areas that are made by forming a gel network and removing the solvent without causing pore collapse. Due to characteristics such as high surface area, high porosity, and low density, these lightweight aerogels are attractive for use as thermal insulators, low dielectric substrates, catalyst supports, and as building and construction materials. A great deal of attention has been paid to investigating alternate aerogel backbones such as polymers and polymer-silica hybrids in order to overcome the fragility and lack of flexibility associated with silica aerogels.
The inherent fragility of silica aerogels has been addressed in several ways. One approach involves the reaction of oligomers containing functional moieties such as styrene, epoxy, or isocyanate with pendant functionalities on the silica backbone such as amines, hydroxyl or vinyl groups that are either covalently linked to a preformed aerogel or attached during oligomerization and gelation. More robust aerogels are obtained but at the expense of the use temperature. Furthermore, while the compressive moduli of, for example, epoxy cross-linked silica aerogels can be quite high (326 MPa), these increases come at the expense of substantial increases in density (0.85 g/cm3).
More recently, polymer aerogels have been fabricated through the formation of either chemically or physically cross-linked networks which avoid the use of silica altogether. Polymer aerogels tend to be more robust than pure silica aerogels and since factors such as polymer chain length and cross-link density can be controlled it is possible to generate a broad spectrum of properties from a few simple monomers. For example, syndiotactic polystyrene aerogels have been produced by allowing hot solutions of the polymer to form physically interlinked semicrystalline domains that act as virtual cross-links upon cooling. These materials were found to be hygroscopic and were attractive sorption and desorption substrates for removing impurities from air and fluids; however, with their lack of covalent crosslinks, and resulting poor mechanical properties, alternative strategies were employed resulting in increased Young's moduli for these materials.
One strategy for improving the mechanical properties of polymer aerogels has been to investigate composites. A recent investigation has shown that the addition of carbon nanotubes to thermo-reversible polystyrene gels results in their homogenous dispersion and the formation of an interpenetrating 3D network of nanotubes and physically bonded polystyrene domains. The net result was no change for the surface areas but an enhancement of the compressive moduli of the materials with values as high as 6.4 MPa at 0.06 g/cm3. Polystyrene aerogels of the same density, but without nanotube reinforcement, had a lower compressive modulus at 4.2 MPa.
While physical cross-links and the formation of nanotube composites, as in the case of polystyrenes, give rise to modest increases in mechanical properties, a superior approach relies on the use of covalently cross-linked oligomers to form an aerogel. An illustrative example of this relies on the use of various di and tri isocyanates of varying geometries reacting with triethylamine and water in order to oligomerize, cross-link, and form polyurea aerogel networks with good mechanical properties. It was demonstrated that at a given density, these materials tended to have higher Young's moduli than their polystyrene and hybrid aerogel counterparts. Young's moduli ranging from 4 to 300 MPa were obtained in the density range of 0.03-0.55 g/cm3. Even at densities of 0.03 g/cm3, half the density of the strongest polystyrene nanocomposite aerogel, covalently cross-linked polyurea aerogels have a higher Young's modulus (7.03 MPa) than the polystyrenes. When compared to epoxy reinforced silica aerogels of similar densities, polyurea aerogels tend to have Young's moduli that are at least twice as large. For example, in the range of 0.19-0.20 g/cm3, reinforced silica has a modulus of 13 MPa while covalently crosslinked polyurea aerogels have moduli around 33 MPa.
While these recent advancements in the areas of organic polymeric aerogels and inorganic hybrids have increased the strength and durability of these materials over pure silica aerogels, low use temperatures limit their utility. A substantial improvement over these earlier technologies was the development of the polyimide aerogels, which exhibit Young's moduli as high as 102 MPa at densities as low as 0.181 g/cm3 making them, as a function of density, more rigid than isocyanate and styrene derived aerogels. The techniques used to fabricate them are simple and easily scalable. However, the use of relatively expensive diamines and dianhydrides coupled with the use of cross-linkers such as 1,3,5-triaminophenoxybenzene and octa(aminophenyl)silesquioxanes, which are not widely available, are limiting factors in their widespread application.